One of the main systems to suffer when you're pushing your stress limits is your immune system. Getting more colds or cold and flu like symptoms is a classic sign, as is suffering from more allergies or intolerances.
However it isn't as cut and dried as that. You see the stress hormone cortisol is a steroid so it suppresses the immune system. Therefore if you're stressed all the time you may suppress any immune symptoms that you should be showing and not realise you're ill when you are.
That might sound odd but if you're running from a tiger (what your stress response is meant for) then it's not a convenient time to be feeling a little peaky!
You'll know if this applies to you because firstly you'll probably be feeling somewhat invincible (party all night, work all day and never get a cold - definitely too good to be true), but also because every time you get any extended period of relaxation such as going on holiday you'll more than likely get sick and feel dreadful as your cortisol levels will have dropped.
If this sounds like you it's time to take action, before you push yourself over the edge. In particular you need to address all sources of stress and work on relaxing - you're probably not the type of person who likes to relax so it may take some effort!